Development of conversion factors to estimate the concentrations of heavy metals in manure-derived digestates

During biogas production, a residual by-product rich in organic matter, nutrients, and trace elements - called digestate - is generated. Due to the nature of the anaerobic digestion process (i.e., conversion of organic matter into biogas) and the non-digestibility of trace elements, metal concentrations are higher in digestate than initially in the treated feedstock, resulting in a detrimental effect on the environment when directly applied as fertiliser on the soil. This study aims to predict the concentration of heavy metals in digestate through four different process parameters (Biogas yield - M₁, Biodegradable fraction - M₂, Dry matter - M₃ and Power generation - M₄) in full-scale biogas plants. For the validation of the process parameters, the predictions were compared against laboratory analyses of feedstocks and digestates samples from mono- and co-digestion processes. The convergence between the conversion factors based on laboratory data and process parameters (C_LD and C_FA, respectively) ranged in the following order: M₃ > M₂ > M₁ > M₄. Based on laboratory analyses, better predictions were obtained for Al, Cr, Cu, Fe, Mn, and Zn employing M₃. Moreover, a robust convergence was achieved between the C_LD and C_FA conversion factors for the mono-digestion process. Further assessment of a diverse range of feedstocks is needed to increase the convergence between the conversion factors based on process parameters and laboratory data, specifically for the co-digestion process M₃. The concentrations of Cd, Co, Ni, and Pb elements were below the detection limits, whereas Cr, Cu, and Zn did not exceed the legal threshold limits of the legislations.